This invention relates to systems, apparatuses, and methods for treating the interface emulsion or “rag” that accumulates at the oil/water interface inside of separation, dehydration, and desalting vessels.
In many industries, including oil, paper and pulp, textiles, and food processing, various processes produce contaminated water as a by-product. This is especially true in crude oil production and refining because substantially all crude oil is produced from subterranean formations which contain water.
The basic method of separating a mixture of oil and water is by use of gravity. For this purpose, separator vessels are frequently employed at the point where the crude oil first reaches the earth's surface. These separators range from rather unsophisticated holding vessels—which simply provide an enclosed container wherein the oil and water mixture can rest with reduced turbulence, thereby allowing the oil to float to an upper part of the vessel and water to settle to a lower part of the vessel—to more sophisticated vessels that apply desalting and dehydration methods, including the use of electrostatic fields in the oil layer of the vessel (see e.g. FIG. 1).
Regardless of the type of vessel used, it is common for oil-coated solids (“mud”) to accumulate in the bottom of the vessel and for a mixture of oil and water (“emulsion” or “rag”) to form at the oil and water interface. The rag layer tends to be a very stable layer that includes, in addition to oil and water, such things as excess chemicals, fine solids, scale, iron sulfides, and other residual particles. If this rag layer is not treated effectively, it can hinder coalescence of water droplets within the vessel and, therefore, compromise the efficiency of the vessel.
Because of the potential for the rag layer to compromise vessel efficiency (and therefore effectiveness), prior art dehydrator and desalter vessels are designed with increased volume to allow for rag layer formation up to a certain maximum height or depth. The accumulated rag layer is then periodically drawn off from the vessel, treated outside the vessel, or in most cases circulated back to the vessel. Shutting down a vessel to withdraw the rag layer disrupts the separation, dehydration, and desalting process, thereby disrupting the crude oil production or refining process, and, in many cases, requires extra vessels to handle production when one or more vessels are shut down.
A need exists for systems, apparatuses, and methods to better control the rag layer in order to keep vessel performance stable within an optimum range and prevent the layer's build-up past the point at which vessel performance is compromised.